RADD 2501 The ORCBS - Radiation Safety - Programs & Guidelines - Radiation Safety Manual

RADD 2501 The ORCBS - Radiation Safety - Programs & Guidelines - Radiation Safety Manual

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THE ORCBS > Radiation Safety > Radiation Safety Manual > Radiation Units Two types of units are used for radiation, units of activity and units of exposure (dose). Units of activity quantify the amount of radiation emitted by a given radiation source. Units of exposure quantify the amount of radiation absorbed or deposited in a specific material by a radiation source. In the world today, two sets of units exist. They are the Special units (Curie, Roentgen, Rad and Rem) and the SI or International Units (Becquerel, Gray and Sievert). In the United States, the Special units must be used as required by Federal law. Therefore, in our discussions the units used will always be the Special units. SI units are defined and described in the appendix on units. Units of Activity The unit of activity for radiation is the Curie, abbreviated Ci. Most laboratory facilities use only millicurie (mCi, 0.001 Ci) or microcurie (uCi, 0.000001 Ci) amounts of radioactive materials, since reliable data can only be obtained using low levels of activity for a given isotope. The Curie is an amount of radioactive material emitting 2.22 x 10 12 disintegrations (particles or photons) per minute (DPM). (The international, or SI, unit for radioactivity is the Becquerel, defined as one disintegration per second.) Activity can be measured with an appropriate radiation detection instrument. Most of these measurements are made with a liquid scintillation counter, gamma well counter or Geiger-Mueller (GM) survey meter with appropriate detection probes. These instruments detect a percentage of the disintegrations and display in counts per minute (CPM). It is important to note that the CPM readings from survey instruments are not the true amount of radiation present, since there are factors which decrease the detection capability of even the most sensitive instruments. Two factors influence
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